Deadlock Prevention using Banker's Algorithm

1. Operating Systems

2. Unit 3
CPU Scheduling:
1. Scheduling Concepts,
2. Performance Criteria,
3. Process States,
4. Process Transition Diagram,
5. Schedulers,
6. Process Control Block (PCB),
7. Process address space,
8. Process identification information,
9. Threads and their management,
10. Scheduling Algorithms,
11. Multiprocessor Scheduling.
Deadlock:
1. System model,
2. Deadlock characterization,
3. Prevention,
4. Avoidance and detection,
5. Recovery from deadlock.

3. Deadlock : System model
• Deadlock is a situation where a set of processes are
blocked because each process is holding a resource and
waiting for another resource acquired by some other
process.
• Consider an example when two trains are coming toward
each other on the same track and there is only one track,
none of the trains can move once they are in front of
each other.
• Deadlock can arise if the following four conditions hold
simultaneously (Necessary Conditions)
• Mutual Exclusion: Two or more resources are non-
shareable (Only one process can use at a time)
• Hold and Wait: A process is holding at least one
resource and waiting for resources.
• No Preemption: A resource cannot be taken from a
process unless the process releases the resource.
• Circular Wait: A set of processes are waiting for each
other in circular form.

4. Understanding Conditions for Deadlock
Mutual Exclusion: When two people meet in the landings, they
can’t just walk through because there is space only for one
person. This condition allows only one person (or process) to
use the step between them (or the resource) is the first
condition necessary for the occurrence of the deadlock.
Hold and Wait: When the two people refuse to retreat and hold
their ground, it is called holding. This is the next necessary
condition for deadlock.
No Preemption: For resolving the deadlock one can simply
cancel one of the processes for other to continue. But the
Operating System doesn’t do so. It allocates the resources to
the processors for as much time as is needed until the task is
completed. Hence, there is no temporary reallocation of the
resources. It is the third condition for deadlock.
Circular Wait: When the two people refuse to retreat and wait
for each other to retreat so that they can complete their task, it
is called circular wait. It is the last condition for deadlock to
occur.

5. Resource-Allocation Graph
• In RAG vertices are two type –
• 1. Process vertex – Every process will be represented as a process vertex. Generally, the
process will be represented with a circle.
• 2. Resource vertex – Every resource will be represented as a resource vertex. It represents as a
box, inside the box, there will be one dot. So the number of dots indicate how many instances
are present of each resource type.
• A set of vertices V and a set of edges E. V is partitioned into two types:
• P = {P1, P2, …, Pn}, the set consisting of all the processes in the system
• R = {R1, R2, …, Rm}, the set consisting of all resource types in the system
• request edge – directed edge Pi  Rj
• assignment edge – directed edge Rj  Pi

6. Example of a Resource Allocation Graph
With A Deadlock With A Cycle But
No Deadlock
• If graph contains no cycles :
• no deadlock
• If graph contains a cycle :
• if only one instance per
resource type, then
deadlock
• if several instances per
resource type, possibility
of deadlock

7. Methods for Handling Deadlocks
• Two ways to ensure that the system will never enter a deadlock state:
• Deadlock prevention
• Deadlock avoidance
• System is in safe state if there exists a sequence <P1,
P2, …, Pn> of ALL the processes in the systems such
that for each Pi, the resources that Pi can still request
can be satisfied by
currently available resources + resources held by
all the Pj, with j < I
That is:
• If Pi resource needs are not immediately available,
then Pi can wait until all Pj have finished
• When Pj is finished, Pi can obtain needed resources,
execute, return allocated resources, and terminate
• When Pi terminates, Pi+1 can obtain its needed
resources, and so on

8. Deadlock avoidance
• Single instance of a resource type
• Use a resource-allocation graph
• Multiple instances of a resource type
• Use the banker’s algorithm

9. Banker’s Algorithm
• The banker’s algorithm is a resource allocation and deadlock avoidance algorithm
that tests for safety by simulating the allocation for predetermined maximum
possible amounts of all resources, then makes an “s-state” check to test for possible
activities, before deciding whether allocation should be allowed to continue.
• Let n = number of processes, and m = number of resources types.
• Available: Vector of length m. If available [j] = k, there are k instances of resource
type Rj available
• Max: n x m matrix. If Max [i,j] = k, then process Pi may request at most k
instances of resource type Rj
• Allocation: n x m matrix. If Allocation[i,j] = k then Pi is currently allocated k
instances of Rj
• Need: n x m matrix. If Need[i,j] = k, then Pi may need k more instances of Rj to
complete its task
Need [i,j] = Max[i,j] – Allocation [i,j]

10. Banker’s Algorithm
• Banker’s algorithm consists of
1. Safety algorithm and
2. Resource request algorithm
• Safety Algorithm: The algorithm for finding out
whether or not a system is in a safe state
can be described as follows:
1) Let Work and Finish be vectors of length
‘m’ and ‘n’ respectively.
Initialize: Work = Available
Finish[i] = false; for i=1, 2, 3, 4….n
2) Find an i such that both
a) Finish[i] = false
b) Needi <= Work
if no such i exists goto step (4)
3) Work = Work + Allocation[i]
Finish[i] = true
goto step (2)
4) if Finish [i] = true for all i
then the system is in a safe state
• Resource-Request Algorithm : Let Requesti be the request
array for process Pi. Requesti [j] = k means process Pi
wants k instances of resource type Rj. When a request for
resources is made by process Pi, the following actions are
taken:
1) If Requesti <= Needi
Goto step (2) ; otherwise, raise an error condition,
since the process has exceeded its maximum claim.
2) If Requesti <= Available
Goto step (3); otherwise, Pi must wait, since the
resources are not available.
3) Have the system pretend to have allocated the requested
resources to process Pi by modifying the state as
follows:
• Available = Available – Requesti
• Allocationi = Allocationi + Requesti
• Needi = Needi– Requesti

11. Bankers Algorithm Example 1
• Consider the following snapshot of a system-
• Answer the following questions using the Banker’s algorithm-
• (i) What is the content of the matrix need?
• (ii) Is the system in a safe state?
• (iii) If a request from process P1 arrives for (0,4,2,0), can the request be granted
immediately?

12. Bankers Algorithm Example
Process Allocation Max Available Need
A B C D A B C D A B C D A B C D
P0 0 0 1 2 0 0 1 2 1 5 2 0 0 0 0 0
P1 1 0 0 0 1 7 5 0 1 5 3 2 0 7 5 0
P2 1 3 5 4 2 3 5 6 1 5 3 2 1 0 0 2
P3 0 6 3 2 0 6 5 2 2 8 8 6 0 0 2 0
P4 0 0 1 4 0 6 5 6 2 14 11 8 0 6 4 2
P1 1 0 0 0 1 7 5 0 2 14 12 12 0 7 5 0
• Step 1-Need =Max-Allocation, make a matrix of need column.(Written in green below)
• Step 2-Check Need <= Available (check individual variables of both corresponding columns).
• If available < need then write same available as in last row. For example for P1- Need B(7)> Available B(5)
(See in red) and so system is not in safe state.
• If Available > Need then write next Process availability as:
• Available= Available + Allocation (written in yellow)
• So safe sequence is (P0,P2,P3,P4,P0)

13. Bankers Algorithm Example 1
• Step3 : To Check if a request from process P1 arrives for (0,4,2,0), can the request be granted
immediately
• Compare Given Allocation with Corresponding Need: For P1 (0,4,2,0)< (0,7,5,0)
• Compare Given Allocation with Corresponding Available: For P1 (0,4,2,0) <(1,5,2,0)
• So new Available= (1,5,2,0)-(0,4,2,0)=(1,1,0,0)
• And so new Allocation= (1,0,0,0)+(0,4,2,0)=(1,4,2,0)
• And so new need for P1=(0,7,5,0) – (0,4,2,0) =(0,3,3,0)
• Now replace the previous values of process P1 with new values:
Process Allocation Max Available Need
A B C D A B C D A B C D A B C D
P0 0 0 1 2 0 0 1 2 1 1 0 0 0 0 0 0
P1 1 4 2 0 1 7 5 0 0 3 3 0
P2 1 3 5 4 2 3 5 6 1 0 0 2
P3 0 6 3 2 0 6 5 2 0 0 2 0
P4 0 0 1 4 0 6 5 6 0 6 4 2
• Step 4: Now repeat Step1 t- Step 2 and find safe sequence.

14. Bankers Algorithm Example 2
• Consider a system with five processes Po through P4 and three resource types A,
B, and C. Resource type A has 10 instances, resource type B has 5 instances, and
resource type C has 7 instances. Suppose that, at time T0 , the following
snapshot of the system has been taken:
Solution:
Available instances of A = Total – Allocated = 10 –
(0+2+3+2+0) = 3
Similarly, Available instances of B = Total – Allocated = 5
– (1+0+0+1+0) = 3
Finally, Available instances of C = Total – Allocated = 7 –
(0+0+2+1+2) = 2
Next, Calculate Need by using the formula
Need = Max – Allocation
………………………A B C
Need for P0 = 7 4 3

15. Bankers Algorithm Example 2
• Now find out any process whose Need can
be satisfied with Available. P1 is one such
process. So, Add P1 to safe sequence <P1>.
• Update the Available by using formula
Available = Available + Allocation of
process added to safe sequence (P1)
So, new Available is 5 3 2
• Next, find out another process whose Need
is less than the updated Available i.e., 5 3 2.
P3 is one such process. Hence, Add P3
to safe sequence <P1, P3>.
• Now, Update Available by adding Allocation
of P3 to current Available.
• Proceed in the same manner. If all the
process can be added to the safe sequence
then the system is in safe state otherwise
not.

16. Bankers Algorithm Example 3
• Considering a system with five processes P0 through P4 and three resources of
type A, B, C. Resource type A has 10 instances, B has 5 instances and type C has
7 instances. Suppose at time t0 following snapshot of the system has been
taken:
A) What will be the content of the Need matrix?
Need [i, j] = Max [i, j] – Allocation [i, j]

17. Bankers Algorithm Example 3
• Is the system in a safe state? If Yes, then what is the safe sequence?

18. Bankers Algorithm Example 3
• What will happen if process P1 requests one additional instance of resource type
A and two instances of resource type C?

19. Bankers Algorithm Example 3